The present invention relates to power MOSFETs and more particularly to semiconductor structures for improving the rated maximum rate in change of voltage (dv/dt) which can be applied across a power MOSFET. The invention relates specifically to improvement in the structure in the gate pad area of a power MOSFET.
Under conditions where a MOSFET is conducting current in a reverse direction, a significant amount of charge is accumulated and stored in the p-n diode formed beneath the gate bond pad. This stored charge has now been identified as a principal limitation in the maximum rate of change of voltage which can be applied across a MOSFET. This limitation is evident during reverse recovery when the charge is dissipated.
In a typical n-channel power MOSFET, the source bond pad is in contact with almost all of the p-diffusion area, so that all of the p-well is effectively clamped to the potential of the source. However, in the region of the gate pad, there is typically an aluminum layer directly over the p-well in contact with the gate, so the source metallization is constrained to contact the p-well only around the edge of the gate pad. Therefore, under reverse recovery conditions, a high density of current can impinge the edge contacts of the source metallization around the gate p-well, resulting in catastrophic failure of the device.
What is needed is a MOSFET device which minimizes the risk of catastrophic failure due to high current density at the edge contacts of the p-well under the gate pad.
The devices described hereinafter are n-channel MOSFETs. It is to be understood by those of ordinary skill in this art that the invention applies equally well to p-channel MOSFETs by substituting the appropriate semiconductor types.